Increased influx of freshwater to the ocean from the ice sheets is a potential forcing for abrupt climate changes. For Antarctica in the present climate, these fluxes chiefly arise from melting below the ice shelves and from melting of icebergs transported by the ocean; both fluxes could increase significantly in a warmer climate. Ice sheet runoff and iceberg calving, in roughly equal shares, currently dominate the freshwater flux from the Greenland Ice Sheet (Church et al., 2001; Chapter 4). In a warming climate, runoff is expected to quickly increase and become much larger than the calving rate, the latter of which in turn is likely to decrease as less and thinner ice borders the ocean; basal melting from below the grounded ice will remain several orders of magnitude smaller than the other fluxes (Huybrechts et al., 2002). For a discussion of the likelihood of these ice sheet changes and the effects on sea level, see the discussion in Chapter 10.

Changes in the surface forcing near the deep-water production areas seem to be most capable of producing rapid climate changes on decadal and longer time scales due to changes in the ocean circulation and mixing. If there are large changes in the ice volume over Greenland, it is likely that much of this melt water will freshen the surface waters in the high-latitude North Atlantic, slowing down the MOC (see Section 8.7.2.1; Chapter 10). Rind et al. (2001) found that changes in the NADW formation rate could instigate changes in the deep-water formation around Antarctica.

The response of the Atlantic MOC to changes in the Antarctic Ice Sheet is less well understood. Experiments with ocean-only models where the melt water changes are imposed as surface salinity changes indicate that the Atlantic MOC will intensify as the waters around Antarctica become less dense (Seidov et al., 2001). Weaver et al. (2003) showed that by adding freshwater in the Southern Ocean, the MOC could change from an ‘off’ state to a state similar to present day. However, in an experiment with an AOGCM, Seidov et al. (2005) found that an external source of freshwater in the Southern Ocean resulted in a surface freshening throughout the world ocean, weakening the Atlantic MOC. In these model results, the SH MOC associated with Antarctic Bottom Water (AABW) formation weakened, causing a cooling around Antarctica. See Chapters 4, 6 and 10 for more discussion about the likelihood of large melt water fluxes from the ice sheets affecting the climate.

In summary, there is a potential for rapid ice sheet changes to produce rapid climate change both through sea level changes and ocean circulation changes. The ocean circulation changes result from increased freshwater flux over the particularly sensitive deep-water production sites. In general, the possible climate changes associated with future evolution of the Greenland Ice Sheet are better understood than are those associated with changes in the Antarctic Ice Sheets.